Adds some debugging code in hopes of tracking crash

No one should be adding a WebContents that is being destroyed. In
theory I would prefer a member on WebContents that was set after
observers are notified, but this is close enough.

BUG=297118
TEST=none
R=avi@chromium.org

Review URL: https://codereview.chromium.org/29823002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230049 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230049